If you were gold when you created the 7 characters, then stopped being gold. You lose one character slot they wont delete the character but if you buy more slots then one of those will replace the one lost for going to silver from gold.
May the Prophets guide us in these cryptic times....
It's also important to keep in mind that 1 character slot is specifically for KDF, and so will not show up as usable when mousing over your Fed banner. So you might have 9, but 1 is exclusively for KDF.
STO is about my Liberated Borg Federation Captain with his Breen 1st Officer, Jem'Hadar Tactical Officer, Liberated Borg Engineering Officer, Android Ops Officer, Photonic Science Officer, Gorn Science Officer, and Reman Medical Officer jumping into their Jem'Hadar Carrier and flying off to do missions for the new Romulan Empire. But for some players allowing a T5 Connie to be used breaks the canon in the game.